1. Field of the Invention
This invention relates to novel low viscosity, liquid crystal substances exhibiting a liquid crystal phase within a broad temperature range up to higher temperatures and also having a positive dielectric anisotropy, and liquid-crystalline compositions containing the same.
2. Description of the Prior Art
Display elements using liquid crystals have come to be broadly used for watches, desk calculators, etc. Such liquid crystal display elements utilize properties of the optical anisotropy and dielectric anisotropy of liquid crystal substances, and the liquid crystal phase includes nematic liquid crystal phase, smectic liquid crystal phase and cholesteric liquid crystal phase. However, display elements utilizing nematic liquid crystals among the above-mentioned phases have been practically used most broadly. Further the display mode of such display elements includes TN type (twisted namatic type), DS type (dynamic scattering type), guest-host type, DAP type, etc. and properties required for the liquid crystal substances used for the respective types are different. At any rate, however, it is preferred that the liquid crystal substances used for these display elements exhibit a liquid crystal phase within as broad a temperature range as possible in the natural world. At present, however, no single compound which satisfies by itself such conditions is present, and it is the present status that substances which are endurable to practical use for the present have been obtained by blending several kinds of liquid crystal compounds on non-liquid crystal compounds. Further, these substances should, of course, be stable to moisture, light, heat, air, etc. and also it is preferred for them that the threshold voltage and saturation voltage required for driving such display elements be as low as possible and their viscosities be as low as possible in order to make the response speed higher.
Now, in order to broaden the liquid-crystalline temperature range toward higher temperatures, it is necessary to use liquid crystal substances having a higher melting point as a component. As compounds for attaining such an object, those expressed by the general formula ##STR3## wherein R represents an alkyl group or an alkoxy group of 1 to 12 carbon atoms, have been known (see U.S. Pat. No. 4,154,697). However, such liquid crystal substances having a higher melting point generally have a higher viscosity and hence liquid crystal compositions containing these also have a higher viscosity; thus the response speed, particularly that at lower temperatures, of liquid crystal display elements which are usable up to higher temperatures such as 100.degree. C. have been liable to be notably retarded.
As improved compounds considerably free from such drawbacks, the present inventors have previously invented 4-[trans-4(trans-4-alkylcyclohexyl)cyclohexyl] substituted benzenes expressed by the general formula ##STR4## wherein R represents H or an alkyl group of 1 to 10 carbon atoms; Y represents H or F; and X represents halogen atom or CN, and applied for patent (Japanese patent application laid-open Nos. Sho 57-64626, Sho 57-114531, Sho 57-154135 and Sho 58-10552.
However, the recent technical advance in liquid crystal display elements has been remarkable and display elements being actuated within a range from further lower temperatures up to further higher ones have been required.
The object of the present invention is to provide liquid crystal compounds which satisfy the above-mentioned requirements i.e. exhibit a liquid crystal phase within a broad temperature range and have a high clearing point and nevertheless a low viscosity.